1. Field of the Invention
This invention relates to a control system for hybrid vehicles having an internal combustion engine and a traction motor as prime movers.
2. Prior Art
Conventionally, a hybrid vehicle having an internal combustion engine (hereinafter simply referred to as "the engine") and a traction motor as prime movers is widely known, and a control system for controlling the prime movers of such a hybrid vehicle has already been proposed, e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 5-229351.
The proposed control system determines the optimum torque at which the maximum engine efficiency is attained, in dependence on traveling conditions of the vehicle, and at the same time detects actual torque generated by the engine for actually driving the vehicle. Then, the control system determines or selects demanded or required torque from the optimum torque and the actual torque. When the optimum torque is selected as the demanded torque, and at the same time the optimum torque is larger than the actual torque, regenerative current is caused to be generated for regeneration of energy (specifically, electrical energy).
When the regeneration of energy is carried out by the traction motor, generally, the amount of regenerative energy becomes larger with an increase in the rotational speed of the traction motor. On the other hand, as the rotational speed of the engine increases, the loss of energy due to engine braking increases. In many cases, therefore, it is desirable that the regeneration of energy is performed by lowering the rotational speed of the engine to a lowest possible value during deceleration of the vehicle, to achieve improved efficiency of regeneration. If the engine rotational speed is lowered to an excessively low value, however, the engine can stall especially at quick deceleration of the vehicle (when the driver quickly and strongly steps on a brake pedal of the vehicle).
The conventional control system, however, does not contemplate the influence of engine braking and engine stalling in controlling the traction motor at quick deceleration of the vehicle. Therefore, there still remains room for improvement in terms of efficient collection of kinetic energy of the vehicle at deceleration as electrical energy.